Thread: Simple Intrest

A sum of money doubles itself in 7 years.In how many years will it become 4 times at SI
(a) 15
(b)21
(c)18
(d)25

Call the sum of money x.

So if it doubles in 7 years then

After 7 years it becomes 2x

2x becomes 4x after another 7 years which is four times the original ammount.

So i would have said that the answer would be 14 years but maybe im missing something.

Originally Posted by Niall101

Call the sum of money x.

So if it doubles in 7 years then

After 7 years it becomes 2x

2x becomes 4x after another 7 years which is four times the original ammount.

So i would have said that the answer would be 14 years but maybe im missing something.

I also tried it the same way.But answer was given as 21.
Any way thank you.

ah yes i see the problem I used compound interest.

simple interest you do no get interest on the interest.

So if sum of money = x

Every 7 years you will get x interest

after 7 years you have x + x

After 14 years you have x + x + x

After 21 years you have x + x +x +x =4x